Open Water Swim Question.

I didn't want to over complicate my post but there is more.  Whenever a gps signal is acquired, after being lost, the watch instantly switches to using gps for distance. However, the ramp to correct distance takes time for enough samples to come in. so, if as an example your average pace is 1 meter/sec (you see that parameter with fit decoders), when a new gps signal is acquired you see something of a slow ramp such as, .03 m_s, .1 m/s, .2 m/s.  this continues for about 30 or more seconds until the watch has finally caught up to your speed. so if you think about it,  for this 30 seconds or more, the watch cut your distance short because in reality you were swimming at 1m/s that whole time. So this error cuts down on distance. Now imagine that the watch keeps loosing and regaining gps signal throughout your swim. every time this happens you lose more distance. that's why I think we end up with less distance. depending on your individual stoke technique, the less frequent the watch loses gps signal, the less error you world have. 

That explains a lot because it is reasonable to expect that the GPS signal is lost or sometimes lost when the hand goes underwater.  But triathletes on longer straight swims in open water have an established pace and not much change of direction and no short legs back and forth so the algorithm does a decent job of keeping up on the distance estimation.

When I switched to the walk app to see how it did measuring distance on that last swim, it estimated way long instead of short because it thought I was walking and used a faster pace to predict distance when the signal was lost.  At least that is how I interpret your explanation as applied here.  

Of the four people actively posting in this thread only one has actually swam in the open water with the Fenix 7 and provided evidence (not claims but real evidence) comparing the F7 with other watches.

Once again here's some evidence of comparison swims.

In the first one it's clear that the Instinct has a much more variable pace than the others - it's the red line but it also has the longest distance. The Instinct 2 has recorded the longest swim using the OWS activity every time I've swum with it. But even then the difference has only ever been around 100m give or take.

And this time with an Epix 'running' in a towed float bag giving the longest distance. The watch being towed ‘running’ in the bag has always given the longest distance. However, overall the distance between the shortest and longest is around 100m.

People reading this thread are free to choose what they want to believe to suit their own beliefs.

Can you share kml files?

I do not have a Fenix 7 but my experience with a Fenix 6 may be relevant to this discussion. I swim in the sea most days April to October. If it is not too windy or wavy, I swim freestyle on a measured 1000m buoyed course parallel to the shore which involves just 4 turns as it is T shaped. Using the OWS activity the distance is always within 5% even though (depending on the tidal current) the swim might sometimes take longer and have a higher number of total strokes. From this I deduce that the OWS distance calculation is largely based on the GPS and not on stroke count or time. 

If it is rough, I swim within the confines of a sheltered bay, which is approximately 40m x 70m. The bay is out the tidal current. I usually make about 25 turns. Based on the time I swim for, my stroke rate and usual distance I swim per stroke, the watch OWS underestimates the distance by about 25%. I assume that the GPS getting an intermittent signal during freestyle swimming is unable to cope with the frequent changes in direction. Indeed zooming in, quite a few of the turns appear to have been short of where I actually turned.

Douglas_W said:

If it is rough, I swim within the confines of a sheltered bay, which is approximately 40m x 70m.

Douglas_W said:

the watch OWS underestimates the distance by about 25%.

I think this is to be expected due to the relatively short legs of the swim. I haven't swum in our local lagoon with the F7/Epix but OWS was recorded ok on the F6 and earlier watches (apart from the 5+). The course is triangular with legs between 150m and 350m around the buoys. This was typical of the result with an F6:

www.strava.com/.../4837100129

Another data point for the OWS for both the Epix and the F7 compared to a F6. It's easy to see from the track that all three watches tracked the same course and only 68m difference between the highest and lowest distance. Of note is the greater variation in speed of the F6 compared to the other two, which suggests the Epix and F7 have better satellite connectivity (or pace management via software) than the F6.

The Epix and F7 are both on All + Multiband and the F6 GPS+GLONASS, all on 1-sec recording.

The point FOR ME (but i think for most people here), is that none of these watch report distances correctly. 

I don't speak for anybody else in this forum (nor would I presume too) but personally, I'd much rather have evidence based fact than prejudice based opinion.

For instance, this morning's swim with an Instinct 2 ST, Epix 2 and F7. Again, the Epix and F7 tracked smoother but the difference in distance was not significant from the Instinct. The course is 2000m long with buoys every 250m. Allowing for tides and wayward navigation the recorded distance is pretty close even if the Instinct has recorded a huge 32m further than the Epix and F7.

Fact supported by evidence. Again.

https://www.strava.com/activities/6998538444